1. Field of the Invention
The present invention relates to an automatic frequency control and automatic gain control devices which are used for a satellite communications system to compensate for frequency error due to frequency instability of an oscillator used for frequency conversion, Doppler frequency deviation caused by movement of the satellite, and a level fluctuation caused by transmission path in radio communications systems such as satellite communications system.
While not limited to such a communication system, the instant invention is directed to the frequency control and level of a central station with respect to many remote stations in point-to-multipoint TDM(Time Division Multiplexing)-TDMA(Time Division Multiple Access) satellite VSAT(Very Small Aperture Terminal) systems.
2. Description of the Prior Art
In satellite communications systems, frequency errors occur due to frequency instability of oscillators used for frequency conversion in an earth station and a transponder, which can deviate frequency acquisition range of a demodulator, as well as give interference to adjacent channels. Furthermore, in satellite communications systems comprised of a central station and several remote stations, to which the present invention is applied, a low cost oscillator is used for the remote station in order to minimize the price of each terminal station in consideration of the economic efficiency of the system, which is synchronized with a reference oscillator with high stability in the central station which generates a transmit frequency. Therefore, a device for compensating for frequency error caused by the Doppler effect is required because it becomes not negligible. Additionally, radio communications systems such as satellite communications, requires an automatic gain control device to keep the input level of the demodulator constant as level fluctuation occurs in the transmission paths.
Many frequency control methods have been used to compensate for frequency errors which occur during a transmission path through satellites. Such conventional methods mainly use one or two pilot frequencies. In Canadian patent No. 1,193,674, entitled, "Two pilot frequency control for communication system," the inventor makes use of two pilot frequencies to compensate for frequency errors introduced during transmission. The invention relates to a method and circuit arrangement for synchronizing the carrier frequency of a reference station with the local oscillator frequency of each of a plurality of remote stations. In this frequency control method, pilot signals of the first frequency and the second frequency are transmitted from a reference station to the remote station, and at the remote station the first filter selects the first frequency and the select filter selects the second frequency. The selected frequencies used to derive the third frequency is related to the difference frequency thereof. Also, the fourth frequency is derived from pilot frequencies and this fourth frequency includes the error frequency introduced during transmission. The third and fourth frequencies are combined so as to derive said frequency control signal for adjusting the frequency of the local oscillator to compensate for said frequency errors.
The conventional frequency control technology, using two pilots, has problems including lowering the efficiency of the satellite system activated and operated with limited bandwidth and power as well as a lower economic efficiency of the system, because the transmit equipment becomes complicated as there must be a separate pilot generation circuit on the transmitter and power and frequency bands are required for transmitting pilots.
In addition, the gain control method, which peak-detects the reference pilot signal level and utilizes it as a negative feedback bias of the variable gain amplifier, has been used in order to compensate for level fluctuation caused by transmission path. This peak detection method has a defect in that the output of the variable gain amplifier is adjusted at a lower level than desired, the signal-to-noise ratio becomes lower and lower, and this can cause the communication to be stopped because the signal detected in the system with the low signal-to-noise ratio becomes the peak value of the noise, and not the desired signal.